Definition
An electrical test instrument used to measure very high resistance values, typically in the millions of ohms, by applying a high DC voltage across the item being tested and measuring the small leakage current that flows. It is used primarily to check the quality of electrical insulation in wiring, motors, generators, and transformers.
Plain English
A specialised meter that checks how good the insulation is on wires and electrical equipment. It does this by pushing a high voltage through the insulation and seeing how much current sneaks through.
Context Anchor
Seen in aircraft maintenance and electrical troubleshooting, especially when checking wiring, generators, motors, and other electrical components for insulation condition.
Derivation
From 'meg-' (meaning million, from the Greek 'megas' for great or large), 'ohm' (the unit of electrical resistance), and 'meter' (a measuring device). The name reflects what it does: measure resistance values in the megohm (million-ohm) range, well beyond what an ordinary ohmmeter can read.
Why Pilots Care
Detects degraded insulation that could lead to electrical shorts, arcing, or system failures in flight.
Analogy
It is like pressure-checking a hose for tiny leaks. The hose may look fine from the outside, but the test shows whether anything is escaping where it should not.
Intuition Check
A megohmmeter is not just a regular ohmmeter with a bigger scale. It uses a higher test voltage to check insulation, so it must be used only where the aircraft maintenance instructions allow it.
Example Sentence 1
The technician used a megohmmeter to check the insulation resistance of the generator windings before returning the aircraft to service.
Example Sentence 2
Before reinstalling the starter, the technician checked the windings with a megohmmeter to ensure no insulation breakdown had occurred.